1. Field of the Invention
The present invention relates to fuel cell electric power generation systems and methods and systems of controlling the same.
2. Description of the Related Art
In a fuel cell power generation system, hydrogen and oxygen supplied to the fuel cell stack are converted into electric power. Many such systems employ a hydrocarbon fuel as the hydrogen source. The fuel must be processed to convert it to a hydrogen rich stream. Various embodiments of fuel processing systems are employed to process the fuel, which commonly comprise a reformer, and other components such as shift reactors and selective oxidizers.
Typical reformers employed in such fuel processing systems include steam reformers, partial oxidation reformers (POX) and autothermal reformers (ATR). In steam reformers, for example, the fuel and steam are directed to one or more reformer tubes containing a steam reforming catalyst and converted into a hydrogen rich reformate stream. The steam reforming process is highly endothermic, and is normally carried out at elevated catalyst temperatures in the range from about 650° C. to about 875° C. Such elevated temperatures are typically generated by the heat of combustion from a burner incorporated in the reformer. The reformate stream is often then delivered to a shift reactor and a selective oxidizer, and then to an anode inlet passage of the fuel cell stack to be consumed in the stack.
A majority of hydrogen in the reformate stream is utilized in the fuel cell stack to generate electrical energy. However, operating the fuel cell system at very high hydrogen utilization (i.e. above 90% to 95%) can result in fuel starvation conditions at some portions of the fuel cell stack. Fuel starvation conditions increase the risk of cell voltage reversal occurring in one or more cells in the stack, which can cause damage to membrane electrode assembly (MEA) components, deteriorating the performance of the affected cells and resulting in shortened service time of the stack.
Additionally, in power generation systems that employ steam reformers, anode exhaust gas is commonly supplied as fuel to the reformer burner. Thus it is desired to maintain a sufficient level of unconsumed hydrogen in the anode exhaust for the burner to supply enough heat to the reforming catalyst to support the desired rate of reaction. An insufficient supply of hydrogen to the burners can result in undesirably low reaction rates in the reformer as well as burner flame-out.
Low hydrogen utilization can also be problematic. When hydrogen utilization is low in the fuel cell stack there is excess hydrogen in the anode exhaust gas, which can result in higher reformer temperatures. Higher reformer temperatures tend to increase the fuel conversion rate in the reformer, however, higher temperatures also increase the concentration of carbon monoxide in the reformate. The concentration of carbon monoxide can increase beyond the capacity of downstream fuel processing components resulting in carbon monoxide “slip” to the stack and poisoning of the anode catalyst. With heavier fuels, higher reformer temperatures can also cause coking and carbon formation on the catalyst. Furthermore, at higher reformer temperatures there is an increased risk of damage to the reformer components, including sintering of the catalyst and thermal stress/damage to reformer components.
Thus, the hydrogen requirements of the fuel cell stack and the fuel processing system should be matched for optimal performance of the power generation system. For power generation systems employing steam reformers, for example, hydrogen utilization of about 80% to 85% in the stack is an optimal utilization range. This helps avoid fuel starvation conditions in the stack and supplies sufficient hydrogen to the reformer burner to maintain the desired steam reforming reaction rate.
U.S. Pat. Nos. 3,585,078 and 5,009,967 describe methods of reformer fuel control for controlling the flow rate of a reformate stream to a fuel cell. Both patents disclose regulating the fuel stream flow to the reformer as a function of the fuel cell current and biasing the fuel stream flow rate as a function of the temperature of the reformer. In general, an output current of the fuel cell stack is monitored and processed to determine an initial fuel stream flow rate set point, and to regulate the flow rate accordingly. Second, an empirical model is employed that provides a desired operating temperature, given the measured current load. The desired operating temperature correlates to estimated hydrogen utilization through the fuel cell stack. U.S. Pat. No. 5,009,967 describes attaining an operating temperature in the reformer correlating to 80% hydrogen utilization in the fuel cell stack. The flow rate of the fuel stream is biased to eventually achieve the desired temperature set point. The two-stage control is complicated, requiring an initial set point and an ultimate set point, and response time could be slow. This is particularly problematic for load-following applications, where the desired fuel cell output varies in response to changing load requirements.
Other control methods involve regulating the fuel stream flow to the reformer based on a predetermined reformate composition as a function of the temperature of the reformer. For example, a predicted or empirically determined reformate composition at the measured reformer temperature can be used to estimate the amount of hydrogen supplied to the fuel cell stack. Typically, such methods use a look-up table of reformate compositions over the operating temperature range of the reformer, calculating a hydrogen supply rate to the stack based on the expected reformate composition at the measured reformer temperature. These control methods have several disadvantages. One disadvantage is that the fuel processing system typically includes more than one component, and the output of each component can vary with process conditions. Thus, the predetermined reformate composition may or may not correspond to the actual reformate condition, depending on the output of other components. Another disadvantage is that the performance of each of the fuel processing components changes over time, increasing the difference between the predicted and actual reformate compositions. This change in performance necessitates updating the look-up tables regularly, or modelling the change in performance in the control system. As a result, such control methods are complicated and generally do not provide a desirable degree of accuracy. Again, these problems are aggravated in load-following applications.
Available means of providing control of hydrogen utilization in fuel cell power generation systems rely on complicated control schemes. It is thus desirable to provide a control system and method with fast response rates and for maintaining hydrogen utilization rate in a desired range.